1. Field of the Invention
The present invention relates to a method and apparatus for channel estimation using a dedicated pilot signal in an orthogonal frequency division multiplexing (OFDM)-based wireless communication system using multi-antenna transmission techniques.
2. Description of the Related Art
An OFDM wireless communication system with multi-input multi-output (MIMO) techniques can increase the channel capacity using a spatial multiplexing technique and improve the reliability of data transmission using a diversity technique. Accurate channel estimation at the receiver is indispensable to maximize the performance of a MIMO OFDM-based wireless communication system. The channel information is often estimated using common pilot signal orthogonally allocated to each transmit antenna in the downlink of a MIMO OFDM-based wireless communication system. However, this technique may suffer from high pilot signaling overhead because the amount of common pilot signal increases in linear proportion to the number of transmit antennas. If the MIMO system uses the same amount of pilot signaling overhead as the single antenna system, it can use reduced pilot signal for each antenna, yielding poor channel estimation (i.e., increase of the mean square error (MSE) in channel estimation) in low signal-to-interference plus noise power ratio (SINR) channel environments.
In order to alleviate this problem, the fourth-generation (4G) wireless communication systems, such as IEEE 802.16m and 3GPP LTE, consider the use of a dedicated (or user specific) pilot signal. In an MIMO system which employs a transmit beamforming technique, the data signal is transmitted through a virtual channel which is generated by multiplying the channel with the beam weight vector for the transmit beamforming. The dedicated pilot signal is also transmitted by means of transmit beamforming with the same beam weight identical as that of data signal. The use of dedicated pilot signal is effective in low-SINR environments (e.g., near the cell boundary) mainly because of the reduction of pilot signaling overhead and the effect of beamforming gain as well.
The statistical characteristics of the virtual channel are different from those of individual antenna channel. Moreover, the amount of resource allocated to the dedicated pilot signal is very small. As a consequence, it may not be easy to estimate the statistical characteristics of the virtual channel from the received dedicated pilot signal. Conventional technologies often employs a linear interpolation (LI) technique because of the simplicity (i.e., it does not require the statistical characteristics of the virtual channel). However, the LI technique may be vulnerable to the presence of interference and noise (e.g., near the cell boundary). It also may not be able to minimize the pilot signaling overhead because it cannot optimally determine the pilot pattern according to the channel environment.
The use of a dedicated pilot signal considered in the 4G wireless communication systems is very effective in low-SINR environments mainly due to the improvement of transmit power gain from the beamforming and the reduction of the pilot signaling overhead. However, it may not be easy to estimate the statistical characteristics of the virtual channel because the dedicated pilot signal is allocated to a specific user using a small amount of resource. As a consequence, the use of statistical estimation techniques (e.g., the minimum mean squared estimation (MMSE)) may not be applicable to the channel estimation with the use of dedicated pilot signal, which may limit the exploitation of the dedicated pilot signal in the channel estimation.